(1) Field of the Invention
The present invention relates to a ferroelectric liquid crystal composition. More specifically, it relates to a ferroelectric liquid crystal composition having quick response properties and a large tilt angle, and to an optical switching element containing this composition.
(2) Description of the Prior Art
Liquid crystal compounds are widely used as materials for display elements, and most of these liquid crystal elements are on TN type display systems and the liquid crystal materials are in the state of nematic phase.
The TN type display system has advantages such as being less fatiguing to the eyes and extremely small consumption of electric power because of being a non-emissive type, whereas it has disadvantages such as slow response and disappearance of display at certain visual angles.
In recent years, this system is being improved in characteristics of flat displays, and in particular, the speed-up of response and the enlargement of visual angle are demanded.
In order to meet these demands, improvements in liquid crystal materials have been attempted. However, as compared with other emissive type displays (e.g., electroluminescence displays and plasma displays), it is apparent that the TN type display system is much poorer in response time and the extent of the viewing or visual angle.
In order that characteristics of the liquid display element such as features of the non-emissive type and small consumption of electric power may be maintained and in order that quick response corresponding to that of the emissive type displays may be assured, it is essential to develop a novel liquid display system in place of the TN type display system.
As one of such attempts, a display system in which the optical switching phenomenon of ferroelectric liquid crystals is utilized has been suggested by N. A. Clark and S. T. Lagerwall (see Appl. Phys. Lett. 36, p 899, 1980).
The presence of the ferroelectric liquid crystals was announced for the first time in 1975 by R. B. Mayer et al. (see J. Phys., 36, p 69, 1975), and from the viewpoint of structure, these crystals belong to a chiral smectic C phase, a chiral smectic I phase, a chiral smectic F phase, a chiral smectic G phase, a chiral smectic H phase, a chiral smectic J phase and a chiral smectic K phase (hereinafter referred to simply as "S.sub.C * phase", "S.sub.I * phase", "S.sub.F * phase", "S.sub.G * phase", "S.sub.H * phase", "S.sub.J * phase" and "S.sub.K * phase", respectively).
In the chiral smectic phase, molecules forms a layer and incline to the surface of the layer, and a helical axis is vertical to this layer surface.
In the chiral smectic phase, spontaneous polarization takes place, and therefore, when a DC electric field is applied to this layer in parallel therewith, the molecules turn round the helical axis in accordance with its polarity. The display element of the ferroelectric liquid crystals utilizes this switching phenomenon.
Nowadays, of the chiral smectic phases, much attention is particularly paid to the S.sub.C * phase.
The display system in which switching phenomenon of the S.sub.C * phase is utilized can be further classified into two types: a birefringence type system using two polarizers and a guest/host type system using a dichoric dye.
Features of these display systems are:
(1) Response time is very short.
(2) Memory properties are present.
(3) Display performance is not greatly affected by visual angle.
Thus, the display systems have the possibility of achieving the high-density displays and are considered to be effectively utilizable in the display element. In addition thereto, in the guest/host type display system, only one polarizing plate is used, and therefore this system has the following excellent feature:
(4) there is no coloring by retardation.
However, also in this display system, there are now many problems to be solved.
In the guest/host type system, transmittances T.sub.on and T.sub.off are transmittances of light at the time of "on" or "off", when the polarizing axis of the polarizer is arranged so as to coincide with a liquid crystal molecular axis at the time of "off". Provided that an angle made by the polarizing axis and a helical axis is denoted by .delta., T.sub.on and T.sub.off are represented by the formulae: EQU T.sub.on =1/2{cos.sup.2 (.delta.+.theta.)e.sup.-a.parallel.(.lambda.)d +sin.sup.2 (.delta.+.theta.)e.sup.-a.perp.(.lambda.)d }( 1) EQU T.sub.off =1/2{cos.sup.2 (.delta.-.theta.)e.sup.-a.parallel.(.lambda.)d +sin.sup.2 (.delta.-.theta.)e.sup.-a.perp.(.lambda.)d }( 2)
wherein a .parallel. (.lambda.) and a.perp. (.lambda.) are absorption coefficients of light by a dichroic dye in the liquid crystal to polarized lights parallel and perpendicular to the liquid crystal molecular axis, respectively, .theta. is a tilt angle, and d is the thickness of a cell [Solid Physics, Vol. 20, No. 7, p. 481-488 (1985)].
It can be understood from the formulae (1) and (2) that when .theta.=.delta.=.pi./4 (45.degree.), the highest display contrast is obtained. In other words, the S.sub.C * composition in which the tilt angle is 45.degree. gives the highest contrast. This angle is just twice as much as the tilt angle (22.5.degree.) of the liquid crystal composition which can provide the highest display contrast in the birefringence type element.
On the other hand, it is indicated by the following formula that a response time is rapidly slow, as the tilt angle is great: ##EQU1## where .tau. is a response time, .eta..sub.o is a standardized inherent rotational viscosity which is independent of a tilt angle, .theta. is a tilt angle, P.sub.s is a spontaneous polarization, and E is a field strength.
Therefore, the response time of the ferroelectric liquid crystal composition used in the existent guest/host element is much slower than in the birefringence type element, and thus the practical ferroelectric liquid crystal materials having the large tilt angle and the quick response times have been relatively unknown till now.
For example, a ferroelectric liquid crystal composition for a guest/host type display element described in Japanese Patent Laid-open Publication No. 22889/1987 and a ferroelectric liquid crystal composition described in Japanese Patent Application No. 192633/1988 are unsatisfactory as follows. In the composition in which the tilt angle is 40.degree. or so, a response time is slow, and in the composition in which the response time is about 200 .mu.sec., the tilt angle is as small as about 30.degree.. These compositions are not practical yet.
In order to obtain the ferroelectric liquid crystal composition having a quick response time and a large tilt angle, it is required that the composition has low viscosity, as understood from the above-mentioned three formulae.
Furthermore, the ferroelectric liquid crystal composition having a large tilt angle can be applied to spatial optical modulators in which refractive index anisotropy is utilized and band pass filters through which the light having a certain wave length is only allowed to pass (see Hugh J. Masterson, Noel A. Clark et al., Annual Meeting Draft in 1988, OPTICAL SOCIETY OF AMERICA, p. 118).
The ferroelectric liquid crystal composition having a large tilt angle is applicable to the above-mentioned fields, and therefore with regard to this type of composition, it is a requirement to additionally improve the response properties and the tilt angle.